In the lead after kid's sum (3,2)
I believe the answer is:
tot up
'sum' is the definition.
('tot up' can be a synonym of 'sum')
'in the lead after kid's' is the wordplay.
'in the lead' becomes 'up' ('up' can be similar in meaning to 'in the lead').
'after' means one lot of letters go next to another.
'kid' becomes 'tot' (tot is a kind of kid).
'up' put after 'tot' is 'TOT UP'.
(Other definitions for tot up that I've seen before include "Reckon" , "Add (some numbers)" , "Summate" , "Do summer job" , "Count" .)